Generally, with growing demand for communication, a Wavelength Division Multiplexing (WDM) system exploiting the broadband characteristics of an optical amplifier is used. In recent years, furthermore, a transmission rate of 100 Gbps per wavelength in the WDM system is becoming mainstream, and the traffic per transmission line is thus increasing.
In an optical transmission system to transmit such large volumes of data, since a failure has a serious influence if it occurs, an improvement in reliability of the entire system is required. Therefore, an optical transmission system including an active line and a standby line is used to improve the reliability of the optical transmission system. In the optical transmission system including an active line and a standby line, when a failure occurs in the active line, it is possible to shorten the loss-of-signal time by switching a transmission line for signal light from the active line to the standby line.
During the line switching, no signal light is inputted for a while to an optical amplifier on the downstream side of a switch for line switching due to switching the transmission line for signal light from the active line to the standby line. Generally, since an optical amplifier in the optical transmission system performs constant output to amplify the optical power (input optical level) of the signal light to a predetermined power, an optical surge occurs in the optical amplifier due to variations in optical power before and after line switching. When an optical surge occurs, it may damage the optical amplifier and an optical receiver on the downstream side in the optical transmission system. Accordingly, a method for performing line switching while suppressing variations in optical power has been proposed (for example, patent references 1 and 2).
As the optical receiver in the optical transmission system, a digital coherent receiver using a digital coherent transmission technique as Digital Signal Processing (DSP) is mainstream. The digital coherent transmission technique is a technique for correcting, by using the optical receiver, waveform distortion that causes deterioration in signal quality such as Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD), in ultrahigh-speed signal transmission at a rate of 100 Gbps or more per wavelength.
Generally, since optical fibers having the same characteristics and distance are not always used for the active line and the standby line, a condition for signal processing in a digital coherent receiver is re-searched for in consideration of the difference in characteristic between the active line and the standby line, in switching from the active line to the standby line.